Diffraction gratings are periodic structures manufactured on transparent or reflective substrates. Their basic property consists in generating angularly separated copies of any incoming light beam. Because this phenomenon is wavelength-dependent light beams of different optical frequencies are also angularly separated. A major application of diffraction gratings is in spectroscopy where the grating acts as the dispersive element that spatially separates different wavelengths. Gratings are also used as the stabilizing feedback element in certain types of laser diode modules, as beam splitters, as fan generators used for machine vision applications, etc.
The recent expansion of fiber optics based telecommunication has created a large demand for devices called Fiber Bragg Gratings (“FBG's”). These devices are used, for example, to separate telecom frequency bands or compensate for optical dispersion in long-haul fiber networks. FBG's are also used as embedded strain sensors for civil engineering or geophysical studies and oil, gas or mining exploitation. FBG's are gratings in the sense that the refractive index of the optical waveguide (typically a fiber) is modulated periodically over some distance. The refractive index modulation is typically produced by exposing the waveguide with a high-intensity UV modulation pattern from the side. This modulation pattern, in turn, is typically created using a phase mask, which is simply a dedicated diffraction grating that produces a modulated UV intensity pattern when illuminated with coherent light of appropriate wavelength.
The current band spacing in fiber optics telecommunication dictates that FBG used as channel add/drop devices have a flat 50-Ghz bandwidth with steep flanks and essentially no side lobes. Moreover, they should not introduce unwanted or uncontrolled optical dispersion. However, imperfections in phase mask periodicity can introduce side lobes and dispersion in FBG's. It is thus desirable to be able to characterize these phase masks to identify unwanted characteristics and/or defects prior to writing FBG's.
A parameter of particular interest in phase masks is the chirp, or grating period variation as a function of position along the mask. For example, linear chirp corresponds to a linearly varying period. The required accuracy of grating period in chirped FBG's can be extremely demanding. For example, some FBG manufacturers require this parameter to be below 5 picometers per centimeter along the grating length. This corresponds to a period stability of 10 ppm per cm. Such demanding specifications means that grating manufacturers should measure their components with a resolution on the order of a few pm/cm.
Understandably, such demanding specifications mean that phase masks should be precisely manufactured. A typical approach to qualifying a Bragg grating phase mask is a functional test: writing FBG's in fibers using the phase mask and measuring the properties of the written FBG. However, this approach can be time consuming and implies the simultaneous control of many process parameters that do not relate directly to the quality of the phase mask.